A known circuit of this type, used in Thomson Chassis TX 91 (Asia), Is shown In FIG. 1. A mains supply voltage source is coupled to a full wave bridge rectifier DP1 for producing a rectified voltage V1 which is smoothed by capacitor C7. The SMPS includes further a transformer LP3 with a primary winding W1, a feedback winding W2 and secondary windings W3 and W4. The voltage V1 is connected in series to the primary winding W1 and to a switching transistor T1.
Control means IP1 including an oscillator OS for generating a pulse-width modulated signal provide a driver voltage V2 at the output 14 for driving the switching transistor T1. The oscillating frequency of the oscillator OS can be adjusted by a resistor R13 and a capacitor C26. As control means IP1 an integrated circuit, e.g. TEA 2261 can be used as described in SGS-Thomson Microelectronics Catalogue 1994, pages 1/9-9/9.
The control means IP1 provide a soft start for a safe start-up after switching on the line power. This is accomplished via a resistor R5 charging slowly a capacitor C14 with a high capacitance which provides the necessary power for the integrated circuit IP1 at pins 15 and 16.
Additionally the SMPS starts with a low oscillating frequency to avoid a current build-up in the switching transistor T1. A current build-up can arise when the energy stored in the primary inductance is not fully transferred to the secondary side before a new conduction period is initiated. This will lead to operation in continuous mode and the switching transistor T1 may leave therefore his safe operating area. To reduce the oscillating frequency during start-up, the SMPS includes a resistor R511 and a diode D9 in series which connect the capacitor C26 with a capacitor C12 which is charged by the feed-back winding W2. The capacitor C12 is not charged up initially when the SMPS is switched on. Therefore, the diode D9 disconnects capacitor C26 from capacitor C12. The operating frequency is then fixed by R13 and C26, which is a low frequency (a few kHz). After a certain time capacitor C12 is charged up and then D9 will be conducting and an additional current will charge C26 via R511, thus the oscillating frequency increases to its normal operating frequency (about 22 kHz). This ensures that the SMPS starts safely in discontinuous mode, i.e. the energy stored in the primary inductance is always fully transferred to the secondary side before a new conduction period of transistor T1 is initiated.
The start-up of this known SMPS is depending on the charge-up time of capacitor C14 via resistor R5, therefore, depending on the voltage value of the AC mains input voltage. This leads to a quite long start-up time at a low mains input voltage.